1. Field of the Invention
The present invention relates to an ultraviolet radiation water treatment system for performing a water purifying treatment by use of ultraviolet radiation.
2. Description of the Related Art
Conventionally, water treatment systems represented by water systems have been operated on the basis of ensuring hygiene by chlorine sterilization.
However, in recent years, there have occurred water system contamination accidents caused by emerging or reemerging pathogenic microbes such as cryptosporidium, giargia and the like.
Further, the mass generation of algae through eutrophication of lakes, dams, and rivers as water system water sources, and increasing pollution by organic matters and the like has caused abnormal odor and taste, coloring disorders, aggregation and deposition inhibition, filtration blockage, leakage into filtered water, and other problems.
Furthermore, there has occurred a problem where chlorine agents to be injected into water for sterilization react with organic matters in raw water, thereby generating harmful by-products such as trihalomethanes (It is the general term showing the total amount of Chloroform, Bromoform, Bromodichloromethane, and Dichlorochloromethane.).
These problems have come to a level not to be controlled by a prior-art water treatment system of the basic treatment process including aggregation and deposition, filtration, and chlorine treatment.
In such circumstances, a sterilization (disinfection) technology by an ultraviolet radiation treatment (hereinafter referred to also as ultraviolet disinfection) has attracted much attention as an alternative sterilization technology to the conventional chlorine sterilization. The ultraviolet disinfection has an advantage that it does not require complicated procedures of chemical injection, and does not generate harmful by-products. For this reason, in water treatment plants and the like, the ultraviolet radiation treatment is adopted in some cases for the purpose of sterilization and oxidation of residual organic matters. However, from the viewpoint of the transmittance efficiency of ultraviolet light, a treatment where ultraviolet light is radiated to filtered water or aggregated or deposited water is carried out in general.
On the other hand, in some cases, ultraviolet radiation is applied to raw water for the purpose of aggregation improvements, elimination of infectiousness of pathogenic microbes such as criptosporidium, and the like. This is a treatment where ultraviolet light is radiated in the place of performing a chlorine sterilization to raw water. As mentioned previously, unlike the chlorine treatment, this treatment does not generate by-products such as trihalomethanes even if ultraviolet light is radiated. Further, ultraviolet light is highly efficient to damage the reproductive power of criptosporidium and eliminate its infectiousness. Therefore, the ultraviolet radiation treatment is employed.
In the water purifying treatment, it is preferable that the reproduction of algae included in raw water is prevented, and it has been confirmed that the ultraviolet radiation treatment is also effective as a treatment to prevent the reproduction of algae.
Meanwhile, the radiation efficiency of ultraviolet light changes with the turbidity and chromaticity of water to be treated. Especially, it is difficult to control the water quality of raw water. Therefore, it is difficult to appropriately maintain the radiation efficiency in the ultraviolet radiation treatment, which has been a problem in the prior art.
In order to solve such a problem, there is proposed a technology for realizing an appropriate ultraviolet radiation control by detecting a turbidity of raw water, and controlling a flow rate of raw water that is made to flow through a water pipe containing an ultraviolet lamp according to the detected turbidity (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-169059). In this document, it is proposed to use ultraviolet radiation in an algicidal treatment of water plankton in water storage basins and the like.
Further, there is also proposed a technology for realizing an appropriate ultraviolet radiation control by use of a particle meter in the place of a turbidity meter (refer to, for example, Shigeo Kimura et. al., “Investigation on Basic Performance Evaluation of Particle Measuring Devices”, Water System Association Magazine, vol. 71, No. 10, pp. 31 to 51, October, 2002). Furthermore, there is proposed an ultraviolet radiation system that controls an output of an ultraviolet lamp by use of a turbidity meter and a particle meter in a system for radiating ultraviolet light into raw water in a water purifying treatment plant (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-188273).
In the ultraviolet radiation system and the ultraviolet radiation treatment method described in the above-described prior-art documents, there are problems as shown below.
In general, algae that cause problems in aggregation and deposition cannot be countermeasured only by radiating ultraviolet light into treated water after the aggregation and deposition treatment of raw water. Thus, in order to use the ultraviolet radiation effect as a countermeasure against algae, it is necessary to radiate ultraviolet light into raw water before the aggregation treatment.
However, the water quality of raw water changes greatly with water sources, fluctuations in meteorological phenomena, and the like. More specifically, the turbidity, the number of microbes, and the concentration of organic matter in raw water change greatly owing to the mass reproduction of algae and rainfalls, and in normal cases, the ultraviolet transmittance decreases as these values increase. As a result, the effect of the ultraviolet radiation cannot be attained sufficiently, and the effects of not only the algae countermeasures, but also the sterilization (disinfection) treatment of pathogens and the like decrease, which is another problem with the prior art.